Silica sand, zircon sand, chromite sand, olivine sand, and the like have been often used in a molding sand (refractory granular material) for forming a mold. To save shortcomings of these sands, recently, use of artificial molding sand has been studied. Examples of the artificial molding sand include those mainly composed of Al2O3 such as mullite-based and alumina-based molding sands. Such a molding sand has good properties such as high refractoriness, low heat-expandability, high crushing-resistance and being spherical. This has led to an increasing demand for it every year. In the field of forming a mold employing a furan self-hardening process with an acid-hardening furan resin, there have been increasing cases of trying to use it. A technique of using an acid-hardening resin in such a molding sand mainly composed of Al2O3, which has already been widely known, is known to have problems.
To solve a problem of decreased mold strength in cases of large and complicated molds formed with kneaded sand that is regulated to take a longtime to start hardening, or in the case of long bench life, JP-A 9-47840 discloses a composition for forming a mold containing a hardening agent containing phosphoric acid and organic sulfonic acid as essential components, where contents of phosphoric acid and organic sulfonic acid are 10 to 85% by weight and 5 to 70% by weight, respectively.
To reduce a content of sulfur atom in a mold, JP-A 2006-247716 discloses a method of producing a mold by adding a binder composition containing a furan resin and a hardening composition containing a sulfur atom and a phosphorus atom at a weight ratio of 0 to 0.7 represented by [sulfur atom content/(phosphorus atom content+sulfur atom content)] to 100 parts by weight of a spherical molding sand that is produced by a flame fusion method at a specific ratio, and hardening the furan resin.
JP-A 57-58948 describes a method of producing a furan mold with a reclaimed molding sand, in which p-toluenesulfonic acid or xylenesulfonic acid is used as a hardening catalyst for a furan resin containing nitrogen.